Spin electronic devices are currently a topic of intensive research for various applications ranging from magnetic recording to voltage controlled oscillators (VCOs).
Typically, a spin electronic device is based on a magnetic multilayer structure comprising a first ferromagnetic layer of fixed magnetization direction, also called polarization layer or pinned layer, a non-magnetic spacer of metallic or insulating material and a second ferromagnetic layer of variable magnetization orientation, also called for this reason free layer. The spacer provides for a magnetic decoupling of the first and second ferromagnetic layers. If the spacer is an insulating layer, a magnetic tunnel junction (MTJ) is obtained whereas if it is a metallic layer one obtains a so called spin valve (SV) structure. Spin valves and magnetic tunnel junctions have received considerable attention, mainly for their giant magneto-resistance (GMR) properties. Giant magneto-resistance arises from spin dependent scattering (SV) or spin dependent tunnelling (MTJ). For example, the resistance of a spin valve is low if the pinned layer and free magnetic layer have parallel magnetization directions, whereas it is high, if they are antiparallel. Reversal of the magnetization of the free layer can be obtained by applying an external magnetic field.
In U.S. Pat. No. 5,695,864, another type of reversal mechanism has been proposed where the magnetization reversal is induced by exchange interaction between the free layer magnetization and conduction electrons flowing through or reflected by the free layer.
According to a first type of device, known as planar spin valve, the magnetization vectors of the polarization layer and the free layer both lie in the plane of the layers. In absence of current flowing through the structure, the magnetization vectors are in a stable configuration, either parallel or antiparallel. However, when a current or a current pulse is applied to the device, the electrons passing through the pinned layer have their spins polarized and interact with electrons of the free layer, causing the magnetization vector thereof to precess about its original axis. Depending upon the amplitude and sign of the current, the respective magnetization directions of the pinned layer and the free layer, and the amplitude and orientation of the external field, if any, the cone precession angle either decreases back to zero, stabilizes at a given value or increases until it eventually reaches 180°, thus reversing the magnetization vector of the free layer.
Another promising device is the so called perpendicular polarizer (PERP). In such a device, by contrast with a planar spin valve or a MTJ, the magnetization direction of the polarization layer is orthogonal to the plane of the layers, while the magnetization of the free layer is kept in-plane. As for the planar spin valve, under certain operating conditions, the spins of the electrons polarized by the pinned layer, cause the magnetization vector of the free layer to precess, in this instance about an axis perpendicular to the plane of the layers. The perpendicular magnetization of the polarization layer is achieved by using materials with high perpendicular magnetic anisotropy, as described by S. Mangin et al. in the article entitled “Current-induced magnetization reversal in nanopillars with perpendicular anisotropy” published in Nature, Vol. 5, March 2006, pages 210-215.
Planar spin torque devices and perpendicular polarizer structures have both been used in the prior art for implementing magnetic memory cells. The transfer of angular momentum from the polarized electrons to the free layer magnetization vector can be modelled as an effective torque which, for sufficient current densities, is high enough for switching the magnetization vector of the free layer. The state of the magnetization vector can be used for storing a binary information.
An example of perpendicular polarizer used as magnetic RAM cell has been described e.g. in U.S. Pat. No. 6,532,164 filed in the name of the present Applicant.
For a planar spin torque device, under application of an appropriate magnetic field, a DC current of polarized electrons may cause a precession of the magnetization vector of the free layer without switching it or dampening its motion. The magnetization vector of the free layer enters then a steady oscillating state. The rotating in-plane component of the magnetization vector induces a modulation of the magnetoresistance which in turn generates an oscillating voltage between the polarization layer and the free layer. The oscillation frequency is at the threshold current close to the ferromagnetic resonance frequency of the free layer, i.e. typically lies in the GHz range. Furthermore, the precession frequency and, hence, the oscillation frequency of the oscillations can be tuned by properly controlling the intensity of the DC current.
A tuneable RF oscillator using a planar spin valve structure, based on this principle has already been disclosed in WO2005/064783. It was also suggested by K. J. Lee et al. in their article entitled “Analytical investigation of spin-transfer dynamics using a perpendicular-to-plane polarizer”, published in Applied Physics Letters 86, 22505 (2005), that a perpendicular polarizer would be suitable for use as microwave source. A tuneable frequency in the range 1-20 GHz has been predicted without application of an external magnetic field.
A major problem affecting spin-transfer torque oscillators, either of planar spin valve type or perpendicular polarizer type, is the large current density required for obtaining and sustaining the voltage oscillations. This current density, also called critical current density, can be expressed for a planar structure as follows:
                              j          c                =                                            2              ⁢                                                          ⁢              e                        ℏ                    ⁢                                                    M                s                            ⁢              τ                                      g              ⁡                              (                                  θ                  =                  0                                )                                              ⁢                      α            ⁡                          (                                                H                  u                                +                                  H                  b                                +                                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                                      M                    s                                                              )                                                          (        1        )            and for the perpendicular polarizer:
                              j          c                =                                            2              ⁢                                                          ⁢              e                        ℏ                    ⁢                                                    M                s                            ⁢              τ                                      g              ⁡                              (                                  θ                  =                                      π                    /                    2                                                  )                                              ⁢                                    H              u                        2                                              (        2        )            where Ms is the saturation magnetization, τ is the thickness of the free layer, Hu is the effective uniaxial in-plane anisotropy field in the free layer, α is the Gilbert damping coefficient and g(θ) is a spin-torque efficiency factor depending upon the angle θ between the magnetization vectors of the free layer and the pinned layer.
The critical current density typically lies about 107 A/cm2. At such current density level, the heat generated by Joule effect and hence the temperature rise may damage the structure or severely deteriorate its characteristics.
It can be seen from expressions (1) and (2) that the critical current density can be lowered by decreasing the saturation magnetization, the thickness and/or the damping coefficient. However, decreasing the thickness and the saturation magnetization would lead to thermal instability whereas low damping coefficients can hardly be achieved with materials currently employed in the aforementioned structures.
The problem underlying the invention is therefore to design a spin-transfer torque oscillator having a low operating current density while exhibiting good thermal stability.